Ceramic dielectric materials have been known for an extended. period of time, and have been widely used as electrical insulators in various electrical devices. A typical, widely used insulator is alumina, which generally has a dielectric constant of 9 to 9.9. Alumina has a high firing temperature, in the order of 1550.degree. C., and is difficult to machine due to a hardness of approximately 9. Oxides of titanium have been added to alumina to soften the alumina, with the resulting material becoming a composite, which leads to other problems.
Oda et al U.S. Pat. No. 4,307,198 discloses low-expansion ceramics having a chemical composition of 2-20 wt % of magnesia
(MgO), 10-68 wt % of alumina (Al.sub.2 O.sub.3 and 30-80 wt % of titania (TiO.sub.2), wherein the major component of the crystalline phase is magnesium-aluminum-titanate phase. These ceramics are made by mixing magnesia, magnesium carbonate and/or magnesium hydroxide; alumina and/or aluminum hydroxide; and anatase-and/or rutile-type titanium dioxide; adding a plasticizer, if necessary to form a shapeable mass; shaping the mass by extrusion, pressing, slip casting or injection molding; drying the shaped body; and firing at 1300.degree. to 1700.degree. C. These low-expansion ceramics are taught to be useful in fields where heat-resistance, thermal shock-resistance, wear-resistance, and corrosion-resistance are required. More particularly, the ceramics are taught to be useful as substrates for automobile exhaust gas purification catalysts; filters for diesel exhaust particulates; industrial or automotive ceramic heat exchangers; engine parts such as pistons, cylinder liners, combustion chambers, auxiliary combustion chambers, turbo-charger rotors or the like; gas turbine parts such as nozzles, rotors, shrouds, scrolls, plenum, burner tail cylinders or the like; heat resistant ceramic materials for receivers of solar energy; refractory materials; and chinawares and porcelains for the chemical industries. There is no disclosure as to the electrical insulating properties of these ceramics.
Albers-Schonberg U.S. Pat. No. 2,165,819 discloses the use of magnesium titanate as an electrical insulator. Berge U.S. patent 2,328,410, Ungewiss U.S. Pat. No. 2,691,088 and Thurnauer U.S. Pat. No. 2,665,219 all disclose magnesium titanate ceramic insulators.
Woditsch U.S. Pat. No. 4,173,485 discloses the preparation of zinc and/or alkaline earth titanates, which may be doped with aluminum, phosphorous, boron and/or silicon, for use as white pigments in lacquers, plastics and paper, and as ferro-electrics. In particular, as shown in Example 2, a pigment of magnesium titanate doped with aluminum may be formed in accordance with the disclosed process.
Das Gupta U.S. Pat. No. 2,741,561 discloses a composition, having little if any variance of dielectric constant with change in temperature, based upon Ca.sub.2 TiO.sub.3, Mg.sub.2 TiO.sub.4, plus ZrO.sub.2 and clay additions.
"Phase Diagrams for Ceramists", 1964 supplement, Levin and McMurdie, published by the American Ceramic Society Inc., 1964, at page 247 presents a phase diagram for the system MgO-Al.sub.2 O-TiO.sub.2, with the indication that between MgAl.sub.2 O.sub.4 and Mg.sub.2 TiO.sub.4 solid solutions are obtained.